How can I create one uv map for multiple objects?
In Blender 2.8 and beyond you can enter edit mode with multiple objects selected.

Before Blender 2.8

Simple answer: "you can't"

Because a UV map pertains to each individual object, and only shows in the UV/Image editor while in edit mode, you cannot combine UV maps, or see multiple maps at the same time. (You can only enter edit mode for one object at a time.)

Longer answer: "you don't have to"

Most likely, your best solution is to select all the meshes (not cameras or lights) and press ctrl+j to join them into one mesh. Now you can unwrap them all as if they were one mesh, because they are.

Tip: You may want to reset the origin after doing this. (ctrl+shift+alt+c) -> "Origin to Geometry"

There are also some add-ons that help with generating UV maps that fit with others in the scene, for creating one large texture for all objects, instead of many different textures for each object. Try pressing ctrl+alt+u and typing "UV: Texture Atlas" in the add-on search box. This might help you.